All some good points, but one of the most important, (at least I think so), is the world reach. I don't know if 3speak, or dtube ever have not available in your region, when it comes to video sharing, I have seen that a lot on video's that are linked in steem post.
You mention Facebook, I don't face book and very rarely visit a facebook page, so a question. How many people in different countries do you visit on a regular basis on face book? Russia? Kenya? Argentina? Poland? Portugal? Bali? Venezuela? Nigeria? Slovakia? Are all those peoples and countries as easily accessible on face book as on steem? I know I get a lot of Not available in your region messages from Youtube when visiting videos posted by people in countries other than America. I know facebook and yt are also world wide, but is seeing and finding post and pages from them as easy on facebook as it is on steem block chain?
People talk about content discovery on SBC, but I have no difficulty finding things from different areas of the world, and with google translate, and many post being made in multiple languages, is that not one of the greatest strength of SBC? If I do a search for anything on google most results are American centric to me, I do not get results often from other countries unless I put the country name in the search.
I think Youtube are very sensitive to controlling who can watch what where as they need to stay friendly with the media companies. If 3speak and dtube break the laws they could find local law enforcement on their door. It's a hard system to fight as the companies have paid politicians to fight their case.
I use Facebook where it is useful, i.e. to stay in touch with some friends and organise events. I don't look too much at stuff from abroad.